Crop diseases are caused by plant pathogenic micro-organisms, (e.g. fungi, bacteria or viruses) which can infect the plant at various stages of development, from the dormant seed to the mature plant. Attack by different pathogens results in widely different diseases, from rapid and large-scale tissue death to long-term chronic infections. Crop pests encompass a wide range of animals, but most are invertebrates including nematodes and arthropods such as insects or mites. These pests feed on plant tissues, with different pests attacking different tissues in different ways. For example, at one extreme nematodes may suck the contents of individual root cells while large insect pests may chew away large areas of foliage.
Leaving aside cultivation practices such as crop rotation and sanitation, much of crop protection has relied on the application of agents (pesticides, which is the term used for agents used against both pests and diseases) that are directly toxic to the pest or disease-causing microbe. For example, pests may be treated using insecticides or nematicides, diseases treated with anti-microbial agents such as fungicides or bactericides. Depending on the site of infection or attack, pesticides may be applied to the crop in a number of ways, including foliar sprays, soil drenches or seed treatments. Regardless of application method, conventional pesticides may act through direct contact with the pest or pathogen, or may be absorbed by the plant and fulfil its function when plant tissues are consumed (e.g. systemic pesticides).
When known pesticides are used as seed treatments the seeds are coated with agents that are designed to inhibit or interfere directly with pathogens or pests and these are dried onto the seeds. Such treatments are mostly aimed at providing direct protection against soil borne pathogens or pests that attack the seed, seedling or roots. In most cases, the observed protection is transient and declines as the protectant is degraded, diluted or localised in the soil and roots as growth progresses.
A disadvantage of known pesticides is that many are also toxic to non-target species, resulting in reductions in biodiversity and even harming beneficial species such as pollinating or predatory insects. In addition there are consumer concerns related to the possible human toxicity of some known pesticides.
Genetic modification has been used as an alternative to pesticides as has Integrated Pest Management (IPM), which combines cultivation practices with the use of pest parasites or predators as a means of biological control. However, each has disadvantages.
A further approach to pest control attempts to make use of plants natural defence systems. Plants respond to a vast range of environmental stimuli. Responses include those that provide protection against pests (e.g. herbivores such as insects) and pathogens (e.g. fungi, bacteria, viruses etc). Plant responses to pest or pathogen attack are brought about by a chain of events that link the initial recognition of the stimulus to changes in cells of the plant that ultimately lead to protection. Thus, in response to wounding and to pest/pathogen challenge, there are local and systemic events induced, with signal transduction pathways occurring at the local site, systemic signal(s) communicating the local events around the plant, and signal transduction pathways occurring in distant cells that are responding to the systemic signal(s).
Plant signalling molecules play a central role in these induced responses to environmental stimuli, since they act as the intermediate molecular signals which link attack to the internal end-effect(s) within the plant. For example, in a variety of plant species, jasmonic acid is known to accumulate transiently following wounding or herbivore attack, and is linked to activation of wound-responsive genes. Another example is during the interactions of plants with pathogens, when salicylic acid is known to increase in quantity and is considered to be a central regulator of local and systemic acquired resistance (SAR) and the activation of defence-related genes associated with disease resistance.
Jasmonic acid (JA) has been applied as an external foliar spray (and also as a root drench) to induce insect-pest resistance in crops such as tomato (Solanum lycopersicum) and grapevine. However, such foliar (and root) applications of JA or its derivatives are prone to cause direct damage to the crop, by causing phytotoxicity for example, and are too expensive to be viable commercially.
U.S. Pat. No. 5,977,060 discloses the use of the Harpin protein of Erwinia amylovora to induce hypersensitive and systemic acquired resistance responses in crops to provide disease protection against viruses as well as protection against soil borne fungi, nematodes and some insects attacking early seedling growth. However, protection provided against insects by a seed soak appears limited to aphids, i.e. sap feeding arthropods. The soak is applied prior to sowing and it appears the plant protection would actually be provided by carry over of the seed soak onto seedlings, effectively applying the protein directly to the seedlings. Harpin also has the disadvantage that it is the result of genetic manipulation which may greatly limit its use in many areas.
The use of jasmonic acid as a seed soak applied to germinating bean and melon seeds is known but only for the purpose of providing protection against fungal disease. The protection afforded to plants was limited and likely to have arisen from the jasmonic acid transferring directly onto germinating seedlings.
WO02055480 relates to the application of coronalon and related compounds in inducing resistance to pathogens including insects. Coronalon is an artificial (chemically synthesised) analogue of coronatine which is an analogue of JA-amino acid conjugates, and which has similar, though not identical, biological activity to JA. However, the coronalon was applied to growing plants and thus this disclosure is little different to a JA foliar spray.
WO0141568 discloses the use of Cis-Jasmone sprays to induce plant volatile emissions that repel insect pests and attract beneficial insects. Again, this is applied directly to growing plants.
Accordingly, the present invention aims to address at least one disadvantage associated with the prior art whether discussed herein or otherwise.